National utility grid companies sets out the strategies and requirements for the connection of power generation equipment to the utility grid. These connection requirements are detailed described in Grid Codes and varies from nation to nation.
One of the topics discussed in the grid codes is the capabilities of a wind turbine when the utility grid experiences a fault. It is essential for the operation of a wind turbine and for a reliable supply of power that a wind turbine can stay connected and synchronized to the utility grid during said grid fault.
System faults are typically short circuits and can be any combination of a three phases and earth short circuits. When a short circuit occur the flow of current results in voltage drops (low voltage situation) throughout the utility grid and the magnitude is dependent of the magnitude of the fault current, the impedance of the short circuit path and the type of short circuit. Further, a low voltage situation may comprise more than one voltage drop e.g. two successive voltage drops.
When a wind turbine experience a utility grid fault the generator speed increases almost immediately as a result of the excessive aerodynamical power that cannot be converted to electrical power. Therefore the aerodynamical power must be reduced drastically throughout the period of utility grid fault.
One method of prior art is to turn the blades of the wind turbine from an operating position to a park position and allow the wind turbine generator to trip offline when the utility grid fault occurs. But as the grid codes typically sets up requirements for a low voltage ride through (LVRT) that requires the wind turbine generator to stay connected, said method of prior art is not compatible with the grid codes.
U.S. Pat. No. 6,921,985 discloses a LVRT system for a wind turbine connected to a utility grid. The blade pitch angle is varied when a low voltage is detected in order to maintain the rotor speed below an over speed trip limit. A problem with the system during a persisting grid fault is that the wind turbine would continue to test the speed trip limit e.g. with the risk of inflicting a wind turbine break down.
It is an object of the present invention to provide an advantageous method of controlling a wind turbine during a utility grid fault in a controllable manner and without the mentioned disadvantage of the prior art.